This was a preliminary notice of a set of experiments, then in progress under very unfavourable circumstances, on the rare occasions when a ray of sunlight was procurable. The production of the phenomenon was traced to the peculiar texture of the membrane covering the cornea. The dependence of the whole on parallax was confirmed by the appearance of the radiant spectrum in precisely the same position with reference to the bright spectrum, when it was produced artificially by interposing a plate of slightly ground glass in the course of the light, so long as this plate was sufficiently near the eye to produce the phenomenon at all. A farther confirmation was given by the fact that, if the eye be near the prism, the appearance is the same on whichever side of the prism the ground glass be placed. The effects of achromatic prisms, of absorption by coloured glasses, and of the use of various kinds of homogeneous light taken from a pure spectrum, were also described.